Soon after the introduction of progestin/estrogen combination oral contraceptives (OCs) pills in 1960, several progestin-only pills (POPS) were introduced. The dose of the progestin in the POPs was made lower than in the combined OCs to minimize the occurrence of amenorrhea resulting from complete ovarian suppression. Consequently, ovulation was inhibited in about half the users of POPs. (The standard POPs primarily depend upon cervical mucus thickening to provide contraceptive protection for those who ovulate). Partly because of the lower progestin dose, the absence of exogenous estrogen and the absence of regular withdrawal bleed, POP users have a much higher rate of unscheduled breakthrough bleeding and spotting than combination OC users. Primarily because of the bleeding problems, POPs are used by only about 1-2% of contracepting women, compared to about 30% using combination OCs.
What is needed is a progestin-containing contraceptive which avoids breakthrough bleeding and spotting problems.